Electronic and electro-mechanical musical organs have always suffered from lack of expression because the tones produced were simply keyed on and off, could be sustained indefinitely with no attack or decay and were rather pure, unchanging tones. In wind-driven pipe organs this problem is solved with a mechanical tremulant that varies the wind pressure at a sub-audible rate imparting a vibrato, or pitch variation, and a tremolo, or volume variation, to the tones of the pipes, thus adding excitement to the sound. Often multiple tremulants were used for separate ranks of pipes.
In addition to the tremulant, another technique to improve the sound of a wind organ is the celeste, where multiple ranks of slightly off-tuned pipes where played together to produce a very low frequency undulation in the sound and a complex cancellation and reinforcement of harmonics. This effect is very similar to the process that makes stars twinkle, or move around and vary in brightness. Different paths through the atmosphere have slightly different delays and cause the light to cancel or add and appear to arrive from a slightly different spot in the heavens. The wave lengths of sound waves are much longer than visible light, so the effects of cancellation and reflection can move the sound around a larger space in a pleasing and exciting way.
In electric organs, the vibrato effect is often imparted electronically; though, this is less than ideal as the sound is too precise and comes from a single speaker. If the celeste effect is attempted by purely electronics means, it suffers the same short-coming of emanating from a single or pair of speakers. In live performance venues, only the listeners in the stereo sweet-spot hear the spatial effects while most listeners, including the musician, miss out. In the case of the pipe organ, the pipes are physically spread; and the sounds come from multiple directions. To achieve both the vibrato and spatial effect for electric organs, it became common to use a mechanically-orbited speaker where the sound sprays in different directions as the sound transducer spins about. Many attempts have been made to capture the sound of the mechanically-orbited speaker by electronic means and reproduced by a stereo-sound system with disappointing results.
As the transducer orbits in a mechanically-orbited speaker; the apparent source of sound, the mouth of the horn, moves toward and away from the listener. As the source moves toward the listener, the pitch would rise; and as the sound moved away, the pitch would fall. This pitch change is due to the Doppler Effect. The sound would reflect from various surfaces in the room, producing the spatial effect. In a happy accident, the cabinet selected for the original commercial models of mechanically-orbited speakers interacted with the orbiting source of sound and added a celeste-like effect. At low speeds the celeste effect dominates over the Doppler Effect and is a highly desired feature of the speaker. Unfortunately, this cabinet construction also limits the sound output of the speaker due to the narrow slots. In efforts to increase the sound output, modifications to the cabinet lost the celeste effect.
While mechanically-orbited speakers have been popular in the past, they suffer from several drawbacks. To produce the vibrato effect over the desired range of musical frequencies, the transducer must spin. The speaker cabinet must be rather large and heavy, making it difficult to transport to live shows. The mechanical parts are delicate, requiring frequent maintenance. Attempts have been made to implement mechanically-orbited speakers with rotary joints to conduct sound signals to the orbiting transducers, but noise from the sliding contact and maintenance issues caused this approach to be abandoned.
Synchronizing multiple mechanically-orbited speakers is difficult, and a single physically rotating transducer has limited sound volume output. Venues have grown in size; and audiences have come to expect a full sound, so many performers resort to placing the orbiting speaker in a sound-isolated location, using a microphone and sound amplification system with multiple speakers. This results in loss of the desirable interaction of the speaker and the listening room.
Because the physical size of the orbiting speaker defines the acoustic performance, smaller and less expensive mechanically-orbited speakers do not achieve the desired musical effect, especially losing the desired frequency modulation by only rotating instead of orbiting.
Often mechanically-orbited speakers have only two speeds and no opportunity to vary the effect without physically modifying the speaker, thus, having a very limited expressiveness. This leads to playing techniques that resort to rapidly switching the drive motors on and off in an attempt to achieve intermediate speeds.
Keyboard players often have two or more instruments, or even one instrument, that can emulate more than one acoustic instrument, such as a synthesizer, that can produce tonewheel organ or piano sounds. Mechanically-orbited speakers have difficulty reproducing piano sounds without coloring and cannot reproduce uncolored piano and orbiting organ sound simultaneously. The traditional solution to this problem has been to add an additional pair of amplifiers and speakers dedicated to the stationary channels, adding weight, size and cost to the speaker system.
There is a need in the art for a speaker system to amplify musical instruments that can produce the desired vibrato, tremolo and celeste spatial effects while being lighter and more rugged for transport, having no moving parts to avoid frequent maintenance, being able to achieve the desired vibrato, tremolo and celeste spatial effects in a low-cost configuration, or being able to be driven at high power levels and having multiple orbiting speakers ganged for higher sound levels, having the ability to vary the musical effect for increased expressiveness, and producing uncolored sound simultaneously with vibrato, tremolo and celeste spatial effects.